Affection of the technology of multiple-input and multiple-output to the vibratory testing

The model of SIMO or SISO is usually used in the traditional vibration testing while the test item is always excited by multiple excitations in the field. The dynamic characteristics of structure (such as resonant frequency, mode shapes and modal damping) which will provide the basis for further analysis are obtained by vibration testing. Since the shaker needs to be mechanically attached to the structure under test, it is nearly inevitable that some sort of interaction will occur between them. It means that the traditional vibration testing or simulation don't have the ability to completely replicate the environment of test structure in the field which will lead to inaccurate results or even incorrect results. This paper offers differences between traditional SIMO vibration testing and MIMO vibration testing and presents some advantage of MIMO vibration testing which will distinctly improve the situation in laboratory and make boundary in the laboratory more similar to the environment in the field. Compared to traditional SIMO vibration testing, the MIMO vibration testing in the laboratory not only can significantly minimize the phenomenon of drops in the excitation forces close to resonant frequencies but also can replicate the boundary situations of the test item in the field

On the inversion of describing functions in nonlinear system analysis

Nonlinear mechanical vibrations are widely found in engineering structures. Traditional linear analysis methods are failing to meet the growing requirements of analysis accuracy. Because of the complexity of nonlinear system, there is no general method for nonlinear system analysis. Based on the describing function method, a polynomial inversion method may be used in nonlinear system identification. Using this method, the measured DF can be inverted into the form of force which has a clearer physical meaning and is easier to use for parameter fit. A MDOF system with two types of nonlinearities at the same location is studied to demonstrate this method